Permanent magnets (PMs) are used in a variety of devices, including traction electric motors for hybrid and electric vehicles. Sintered neodymium-iron-boron (Nd—Fe—B) permanent magnets have very good magnetic properties at low temperatures. However, due to the low Curie temperature of the Nd2Fe14B phase in Nd—Fe—B permanent magnets, the magnetic remanence and intrinsic coercivity decrease rapidly with increased temperature. It is known that the substitution of Dy for Nd or Fe in Nd—Fe—B magnets results in increases of the anisotropic field and the intrinsic coercivity, and a decrease of the saturation magnetization (C. S. Herget, Metal, Poed. Rep. V. 42, P. 438 (1987); W. Rodewald, J. Less-Common Met., V111, P77 (1985); and D. Plusa, J. J. Wystocki, Less-Common Met. V. 133, P. 231 (1987)). It is a common practice to add the heavy RE metals such as dysprosium (Dy) or terbium (Tb) into the mixed metals before melting and alloying. However, Dy and Tb are very rare and expensive. Heavy REs contain only about 2-7% Dy, and only a small fraction of the RE mines in the world contain heavy REs. The price of Dy has increased sharply in recent times. Tb, which is needed if higher magnetic properties are required than Dy can provide, is much more expensive than even Dy.
Typical magnets for traction electric motors in hybrid vehicles contain about 6-10 wt % Dy to meet the required magnetic properties. Conventional methods of making magnets with Dy or Tb result in the Dy or Tb being distributed in the grains and in the phases along grain boundaries within the magnet. Nd—Fe—B permanent magnets can be produced using a powder metallurgy process, which involves melting and strip casting, hydrogen decrepitation (hydride and de-hydride), pulverizing (with nitrogen jet milling), screening, and mixing alloy powders for the desired chemical composition. A typical powder metallurgy process is as follows: weighing and pressing under a magnetic field for powder alignment (vacuum bagging), isostatic pressing, sintering and aging (e.g., about 5-30 hrs, at about 500-1100 C, in vacuum), and machining to magnet pieces. Finally, the magnets are surface treated by phosphating, electroless nickel (Ni) plating, epoxy coating, or the like (if needed).
The ideal microstructure for sintered Nd—Fe—B based magnets is Fe14Nd2B grains perfectly isolated by the nonferromagnetic Nd-rich phases (a eutectic matrix of mainly Nd plus some Fe4Nd1.1B4and Fe—Nd phases stabilized by impurities). The addition of Dy or Tb leads to the formation of quite different ternary intergranular phases based on Fe, Nd and Dy or Tb. These phases are located in the grain boundary region and at the surface of the Fe14Nd2B grains.
Dy or Tb (or their alloys) coated Nd—Fe—B powders are used to make the magnet, which results in a non-uniform distribution of Dy or Tb in the magnet microscopically. For example, the amount of Dy and/or Tb can be reduced by about 20% or more compared to conventional processes, or about 30% or more, or about 40% or more, or about 50% or more, or about 60% or more, or about 70% or more, or about 80% or more, or about 90% or more, depending on relative amount of surface powder to core powder and the Dy or Ty concentration in the surface powder, sintering schedule (which affects diffusion of Dy or Ty into the bulk from grain surface). The process involves coating the Nd—Fe—B based powder used to make sintered Nd—Fe—B permanent magnets with Dy or Tb metals or alloys. The Nd—Fe—B based powder can be coated via physical vapor deposition (PVD).
Accordingly, there is a need for improved methods of making permanent magnets, and in particular, Nd—Fe—B based magnets.